Chemotherapy is of prognostic significance to metaplastic breast cancer

This study aimed to evaluate the significance of chemotherapy (CT) among metaplastic breast cancer (MpBC), and to compare the survival outcomes between triple negative MpBC (MpBC-TNBC) and triple negative invasive ductal carcinoma (IDC-TNBC). SEER database was indexed to identify female unilateral primary MpBC diagnosed from 2010 to 2017. Patients were classified into neoadjuvant chemotherapy (NAC) with response (NAC-response), NAC-no response, adjuvant chemotherapy, and no CT. Breast cancer-specific survival (BCSS) and overall survival (OS) was estimated using the Kaplan–Meier method and compared by log-rank test. Cox regression was used to evaluate the independent prognostic factors. A 1:4 propensity score matching method was adopted to balance baseline differences. Altogether 1186 MpBC patients were enrolled, among them 181 received NAC, 647 received adjuvant CT and 358 did not receive any CT. Chemotherapy was an independent favorable prognostic factor. NAC-response and adjuvant CT had a significant or an obvious trend of survival improvement compared with NAC-no response or no CT. MpBC-TNBC was an independent unfavorable prognostic factor compared with IDC-TNBC. Among them, there was significant or trend of survival improvement among all TNBCs receiving NAC or adjuvant CT compared with no CT. Chemotherapy was of important significance to MpBC prognosis and should be integrated in comprehensive treatment for MpBC.


Factors associated with chemotherapy among MpBC patients
As the significance of chemotherapy for MpBC was still somewhat controversial, the factors associated chemotherapy among MpBC were then explored.Variables with statistically significant difference (P < 0.05) in the one-way logistic regression associated with chemotherapy were younger age, non-white race, higher histologic grade or stage and radiation therapy.Based on the multivariate logistic regression model, age less than 60 years, histologic grade II-III, stage II-III and radiation therapy were independently correlated with chemotherapy (Table 2) (Hosmer Lemeshow P = 0.123).

Survival outcomes stratified by chemotherapy in MpBC
After a median follow-up of 48 months (1-119 months), 321 MpBC patients died, among whom, 239 patients died due to breast cancer.There were statistically significant differences in BCSS and OS among MpBC patients with NAC-response, NAC-non response, adjuvant CT or without CT (P < 0.001) (Fig. 1).According to COX multivariate analysis, chemotherapy was the independent prognostic factor for both BCSS (P = 0.009) and OS (P < 0.001).Compared with no CT, NAC-response and adjuvant CT had a significant or an obvious trend of survival improvement (no CT as reference, HR for NAC-response was 0.691 (0.444-1.077) for BCSS and 0.479 (0.321-0.715) for OS; HR for adjuvant CT was 0.658 (0.480-0.902) for BCSS and 0.451 (0.346-0.587) for OS), while NAC-no response did not improve survival outcomes compared with no CT (no CT as reference, HR for NAC-no response was 1.266 (0.744-2.155) for BCSS and 0.984 (0.610-1.587) for OS) (Table 3).
There were significant differences in survival outcomes among NAC, adjuvant CT or no CT in the subgroup analyses when stratified by stage.Among MpBC patients in stage I, there were only six cases with NAC-response.Although a similar BCSS was observed between adjuvant CT and no CT (P = 0.588), those with adjuvant CT did have an improved OS (P = 0.017) (Fig. 2a,b).Among MpBC patients in stage II, those with NAC-response or adjuvant CT had a significant improved BCSS and OS compared with NAC-no response or no CT (Fig. 2c,d).Among MpBC patients in stage III, chemotherapy lost its prognostic significance as patients with NAC-response, NAC-no response, adjuvant CT and no CT had similar BCSS in most comparisons (Fig. 2e).However, chemotherapy still improved OS as patients with NAC-response and adjuvant CT had a significant or trend of improved OS compared with those with NAC-no response or no CT (Fig. 2f) (Table 4).
When stratified by chemotherapy types, MpBC-TNBC and IDC-TNBC had similar survival outcomes among those with NAC-response and adjuvant CT (Fig. 4a-d).Among those with NAC-no response, IDC-TNBC had significant improved BCSS and OS compared with MpBC-TNBC (Fig. 4e,f).Among those with no CT, IDC-TNBC had a similar BCSS but an improved OS compared with MpBC-TNBC (Fig. 4g,h).

Discussion
Evidence on treatment strategies for MpBC is limited, as management of MpBC is largely paralleled that of IDC and adopts a comprehensive therapy including surgery, chemotherapy, radiotherapy, endocrine therapy, and targeted therapy based on clinical-pathological characteristics and tumor stage.In particular, the efficacy of adjuvant chemotherapy and neoadjuvant chemotherapy is still controversial.Our study was among the largest population-based study to explore the prognostic significance of chemotherapy among MpBC receiving adjuvant CT, NAC or not any CT, and to compare the long-term survival difference between MpBC-TNBC and IDC-TNBC based on PSM.www.nature.com/scientificreports/MpBC generally has aggressive clinical and pathological features.The clinical-pathological characteristics of the cohort of MpBCs in the current study was in line with those reported in the literature 1,2,7,9-14 , in which most MpBCs were presented in larger tumor, higher histologic grade, higher number of positive lymph nodes and majority of TN phenotype.In this study, 82.5% MpBC cases had histologic grade III, 19.6% had positive lymph nodes, 21.2% had T3 or T4 disease, and 13.3% were in stage III.Besides, 71.2% cases were in TN phenotype, which was consistent with previous studies 2,14 .The rate of HER2 overexpression (5.7%) and positive hormone receptor (HR) status (24.9%) was in accord with previous reports 15,16 .However, HR and HER2 status remained no impact on prognosis of MpBC.In patients with or without HER2 overexpression, the prognosis of single HR + tumor was similar to single HR + or double HR-tumor 15 .The role of HER2 in MpBC patients remains unclear 16,17 .Based on multivariate analysis in this study, molecular subtype (TN as reference) was not an independent prognostic factor of BCSS or OS for MpBC.
In spite of high proportion of aggressive characteristics, the effectiveness of standard chemotherapy regimens for MpBC was controversial, as in most studies MpBC was considered in part chemo-resistant [18][19][20] .The poor response to anthracyclines and taxanes suggested chemoresistance probably associated to epithelial-mesenchymal transition (EMT) 5,10,21 which was frequently observed upregulated in these tumors [22][23][24] .Despite the traditional notion that MpBC is resistant to chemotherapy, systemic chemotherapy is administered to 53. 4  2 .Several studies also conducted prognostic nomograms for predicting the OS for MpBC, in which chemotherapy was a favorable prognostic factor [26][27][28] .
The role of chemotherapy in MpBC has been confirmed in this study, and the potential subgroups benefiting from CT was also explored.MpBC patients who received adjuvant CT and NAC with response had an improved BCSS and OS compared with those without CT.Due to limited cases, patients with NAC-response only had an obvious trend of BCSS improvement.However, the HR value in the multivariate analysis was similar to that of adjuvant CT group, indicating that it reduced the death risk to the same extent.Among patients in stage I, those with adjuvant CT did not show significant survival benefit compared with those without CT.It could be postulated that surgery still remained to be the standard therapy in most early-stage MpBC case such as stage I, which had a favorable prognosis and a radical surgery might be adequate for cure with systemic therapy exempt safely.Likewise, according to Chen's study, among node-negative MpBC, CT improved the prognosis of T1c MpBC patients but not T1a and T1b patients to a beneficial extent 29 .Meanwhile, among locally advanced disease such as stage III, patients with adjuvant CT, no CT, NAC with or without response had similar BCSS in most cases.However, CT showed OS improvement compared with no CT or NAC-no response in stage I and stage III.It was suggested that when MpBC progressed to an advanced stage, CT might have limited benefit for  21,30 .However, only among stage II disease for which systemic therapy was essential, patients with adjuvant CT or NAC with response had better prognosis than those without CT or receiving NAC without response.It could be postulated from our study that chemotherapy should be included as the multi-disciplinary treatment for MpBC patients with high-risk features, and early screening was also of first-priority for MpBC.One of the strengths of this study was that it distinguished the response to NAC to explore respectively the significance of NAC for MpBC.Although the response to NAC can predict clinical outcome, there is a dearth of studies evaluating response to NAC in MpBC.In this study, 15.3% MpBC patients received NAC while 54.6% received adjuvant CT.A study from the European Institute of Oncology revealed that just 7.8% of MpBC received NAC and the majority undergoing adjuvant CT 31 .An earlier NCDB study demonstrated that NAC was used in only 15.5% of patients with MpBC 1 .MpBC has been considered poorly responsive to NAC.Previous small case series demonstrated pathological complete response (pCR) rates of approximately 10%, substantially lower than that of classic IDC 32,33 .As a result, some argued that MpBC should not receive NAC 31,32 .In this study, only 12.2% MpBC patients receiving NAC achieved CR while 75.7% showed response to NAC.According to multivariate analysis, NAC-response showed an obvious improvement for BCSS and OS, just like adjuvant CT.However, NAC-no response could not improve survival outcomes.Based on Haque's study, there was significantly improved 5-year OS among MpBC patients with pCR 34 .It suggested that CT had important prognostic significance for MpBC and the response to NAC could help select favorable subsets which may experience long-term favorable prognosis 14,35,36 .Further researches are warranted to explore biomarkers to ensure appropriate patient selection 37 .
Although the majority of MpBC is presented with TN phenotype, the survival difference between MpBC-TNBC and IDC-TNBC is still controversial.Many retrospective studies with small sample size agreed that the prognosis of MpBC-TNBC was significantly worse than that of IDC-TNBC 2,3,7,13 , while other research indicated that these two had similar overall and disease-free survival 31,38 .Larger studies documented a significant worse prognosis of MpBC-TNBC than other IDC-TNBC from the NCDB database, and the significant survival difference was maintained at multivariable analysis.As MpBC tended to present with more locally advanced disease in comparison to IDC-TNBC 20 , PSM was adopted to balance the baseline differences in this study.Yet MpBC was confirmed as an independent unfavorable prognostic factor compared with IDC-TNBC based on multivariate COX regression after a successful PSM.Furthermore, chemotherapy was also a favorable prognostic factor for BCSS and OS among MpBC-TNBC and IDC-TNBC based on the multivariate analysis in this study.Subgroup analysis indicated that MpBC-TNBC had similar survival outcomes compared with IDC-TNBC when they received adjuvant CT or NAC with response.It suggested that chemotherapy was of most importance to these two aggressive subtypes.The current standard of care for MpBC follows the same guidelines as IDC-TNBC.According to Polamraju's study, CT was associated with improved OS among MpBC and IDC-TNBC 3 .On the contrary, IDC-TNBC had significant improved BCSS and OS compared with MpBC-TNBC when they receiving NAC but with no response, and it still had an improved OS compared with MpBC-TNBC when they did not receive CT.It further suggested that the histology of MpBC might confer an additional survival disadvantage.Mutations in PIK3CA, PIK3R1, ARID1A, FAT1, and PTEN were more frequently harbored in MpBC in comparison to IDC-TNBC, which may contribute to the poor clinical outcomes in MpBC 39,40 and warrant further research.
The strengths of this study were obvious, such as large sample size, classification of chemotherapy types of NAC-response, NAC-no response, adjuvant CT and no CT in all analyses, and comparison with IDC-TNBC based on PSM.However, some limitations should also be addressed.Firstly, although chemotherapy was confirmed of great significance to MpBC, the chemotherapy regimens, duration and response was unavailable in the SEER database.Secondly, MpBC has been shown to be extremely heterogeneous in morphology and in survival outcomes 17,41 .However, in this study, all MpBC cases together with the special subtypes were included, www.nature.com/scientificreports/and chemotherapy was confirmed as an independent favorable prognostic factor for BCSS and OS.Lastly, the intrinsic bias could not be avoided in spite of the large sample size.
In conclusion, chemotherapy was of important significance to the prognosis of MpBC and should be integrated in the comprehensive treatment for MpBC.Further researches are warranted to explore the potential biomarkers in MpBC to predict response to chemotherapy.

Patient cohort and stratification
The patient population in this study used data derived from the Surveillance, Epidemiology, and End Results (SEER) database released in 2021.Female unilateral primary MpBC of no special type (MpBC-NST) (coded as 8575) between 2010 and 2017 were enrolled.Besides, some special subtypes of MpBC were also collected, that was, spindle cell carcinoma (coded as 8032), squamous cell carcinoma (8070), low-grade adenosquamous carcinoma (8560), sarcomatoid carcinoma (8033), MpBC with chondroid differentiation (8571), fibromatosis-like MpBC (8572) and myoepithelial carcinoma (8982).Invasive ductal breast cancer with triple negative subtype (IDC-TNBC) which met the inclusion criteria above were also enrolled for comparison with MpBC-TNBC.Patients who had more than one primary cancer, metastasis disease at diagnosis or no surgery performed or no record of surgery, who were diagnosed at death or autopsy alone, missing during follow up or less than 12 months follow-up without death event were excluded.Patients with unknown race, histologic grade, T or N category, ER or PR or HER2 status were also excluded.Histologic grade III was defined as poorly differentiated and anaplastic histologic grades disease.CT status 'yes' together with response information to neoadjuvant therapy was defined as neoadjuvant CT (NAC), among which response to NAC stated as 'complete response' , 'partial response' and 'response to treatment, but not noted if complete or partial' was defined as 'NAC-response' , while 'no response' was defined as 'NAC-no response' .CT status 'yes' together with 'systemic therapy after surgery' was defined as adjuvant CT.CT status 'no or unknown' together with no systemic therapy was defined as 'no CT' .The patient cohort selection process and study consort diagram were shown in Fig. 5.

Table 3 .
Multivariate COX regression of independent prognostic factors for MpBC.*American Indian/AK native, Ascian/Pacific Islander.significant survival improvement.Perhaps the limited cases in stage III might restrict the statistical efficacy to tell the difference.Several studies have reported that the effect of CT associated with better outcome was limited in early-stage cases Vol.:(0123456789) Scientific Reports | (2024) 14:1210 | https://doi.org/10.1038/s41598-024-51627-1www.nature.com/scientificreports/ Figure 2. Subgroup analyses of Kaplan-Meier survival curves of BCSS and OS in MpBC based on tumor stage stratified by chemotherapy types ((a) KM curves of BCSS in stage I; (b) KM curves of OS in stage I; (c) KM curves of BCSS in stage II; (d) KM curves of OS in stage II; (e) KM curves of BCSS in stage III; (f) KM curves of OS in stage III).Vol:.(1234567890)Scientific Reports | (2024) 14:1210 | https://doi.org/10.1038/s41598-024-51627-1www.nature.com/scientificreports/

Table 4 .
Pairwise survival comparisons among different chemotherapy types for MpBC stratified by tumor stage.Kaplan-Meier survival curves of BCSS and OS stratified by MpBC-TNBC and IDC-TNBC ((a) KM curves of BCSS; (b) KM curves of OS).

Table 5 .
Multivariate COX regression of independent prognostic factors of MpBC-TNBC and IDC-TNBC.*American Indian/AK native, Ascian/Pacific Islander.Subgroup analyses of Kaplan-Meier survival curves of BCSS and OS based on chemotherapy types stratified by MpBC-TNBC and IDC-TNBC ((a) KM curves of BCSS in NACresponse; (b) KM curves of OS in NAC-response; (c) KM curves of BCSS in adjuvant CT; (d) KM curves of OS in adjuvant CT; (e) KM curves of BCSS in NAC-no response; (f) KM curves of OS in NAC-no response; (g) KM curves of BCSS in no CT; (h) KM curves of OS in no CT.